Outlook 2010 calendar invite error

One of my users getting this error below when someone sends a calendar invite to her. She cannot accept this invitation. She also cannot make any changes on her recurring meetings. If she does, she will get the error message below. We do have other users who have more mailbox/calendar used space than her and not getting this error but her.

Error: "The message store has reached its maximum size. To reduce the amount of data in this message store, select some items that you no longer need, permanently delete them"